// Copyright 2012 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "src/base/once.h"

#ifdef _WIN32
#include <windows.h>
#else
#include <sched.h>
#endif

namespace v8 {
namespace base {

    void CallOnceImpl(OnceType* once, std::function<void()> init_func)
    {
        // Fast path. The provided function was already executed.
        if (once->load(std::memory_order_acquire) == ONCE_STATE_DONE) {
            return;
        }

        // The function execution did not complete yet. The once object can be in one
        // of the two following states:
        //   - UNINITIALIZED: We are the first thread calling this function.
        //   - EXECUTING_FUNCTION: Another thread is already executing the function.
        //
        // First, try to change the state from UNINITIALIZED to EXECUTING_FUNCTION
        // atomically.
        uint8_t expected = ONCE_STATE_UNINITIALIZED;
        if (once->compare_exchange_strong(expected, ONCE_STATE_EXECUTING_FUNCTION,
                std::memory_order_acq_rel)) {
            // We are the first thread to call this function, so we have to call the
            // function.
            init_func();
            once->store(ONCE_STATE_DONE, std::memory_order_release);
        } else {
            // Another thread has already started executing the function. We need to
            // wait until it completes the initialization.
            while (once->load(std::memory_order_acquire) == ONCE_STATE_EXECUTING_FUNCTION) {
#ifdef _WIN32
                ::Sleep(0);
#else
                sched_yield();
#endif
            }
        }
    }

} // namespace base
} // namespace v8
